bo2 Bulletin American Museum of Natural History (Vol: LITI 
which eventually form islands. Thus the Congo itself in the equatorial 
bend reaches from 6 to 16 kilometers in width. Its shores are so low 
that they often are hard to define, especially at high water when much 
of the surrounding forest is inundated and almost the whole region 
between the Mongala and Tshuapa-Busira Rivers forms one continuous, 
- wooded swamp. The difference between the mean level of the Congo at 
Stanleyville and Leopoldville, a distance of over 1,500 kilometers, 
is less than 200 m. Between the mouth of the Aruwimi and that of the 
Ubangi, the stream flows through a maze of low islands, many of con- 
siderable size; that of Sumba, opposite the mouth of the Mongala, is 80 
kilometers long, while the neighboring island of Ukaturaka, the largest 
of all, has a length of 100 kilometers, but is separated from the mainland 
by only a narrow channel. Moreover, there are frequently broad ex- 
panses of open water, sometimes 500 m. to 2,000 m. wide. ‘There can be 
little doubt that this central, swampy part of the Congo basin was 
formerly occupied by a vast fresh-water lake, which most probably per- 
sisted throughout the Pleistocene and was gradually drained off across 
the Crystal Mountains to the Atlantic. Lake Tumba, Lake Leopold 
II, and Stanley Pool may be regarded as the last remnants of this 
equatorial lake. | 
The most interesting hydrographic feature of the Congo is the abrupt 
transition between the raised periphery or rim of the basin and its central, 
flattened bowl. A glance at the hydrographic map (Map 15) discloses 
that, after having traversed fairly level highlands, most tributaries 
suddenly enter a stretch of rapids, cataracts, or falls, where they drop 
rapidly from the level of the peripheral peneplain to that of the central 
bowl. The Congo itself does so, first in its upper part, the Lualaba, 
where it is frequently interrupted by reaches of rapids, and finally before 
reaching its estuary, when it forces its tortuous way across the peripheral 
rim in the district of the Cataracts. In this last, short stretch of 280 
kilometers, between Stanley Pool and Matadi, the total fall is about 274 
m., considerably more than in the whole distance between Stanley 
Falls and Stanley Pool. The abrupt transition from the peripheral 
peneplain to the equatorial bowl is especially well marked on the Kasai 
and its many affluents. Between 5° and 6°S., the course of all of them, 
with the exception of the Loange, is interrupted by a series of rapids. 
~ It has been shown that this belt of cataracts is due to a fault line which 
may be traced from the estuary of the Congo to the Upper Lualaba. 
The southern edge of this fracture is raised as a scarp about 200 m. above 
the country to the north. 
